1. Field of the Invention
The present invention relates generally to vehicle wheels having a rim for supporting an pneumatic tire and more particularly to an improved rim configuration for reducing rim failure in the event of tire failure.
2. Description of the Related Art
When a vehicle tire fails during vehicle operation, the tire is sometimes destroyed and the vehicle may continue to move on the rolling rim. It is important in such cases that rim integrity be maintained until the vehicle is brought safely to a stop. Aircraft wheels are required by FAA TSO-C26c to pass a roll-on-rim test as part of their qualification. This test consists of rolling a wheel without a tire at a specified speed and load for a specified minimum distance without fragmentation of the wheel. In practice, this has proven to be a difficult test to pass, partly due to the intensity of the cyclic stresses in the area where the rim flange is in contact with the dynamometer or runway. The local stresses in the contact area can be reduced by decreasing the flange load. The total applied load and the axle/wheel stiffness distribution generally control the flange load. In the case of a cantilever mounted wheel, the inboard flange typically carries a greater share of the total applied load than does the outboard flange due to axle deflection. This load share can be altered, within limits, by the use of differential flange heights, that is, some combination of a decreased inboard flange height and/or increased outboard flange height. This technique is described in Baird et al U.S. Pat. 4,275,932 entitled NON-FRANGIBLE WHEEL and is frequently used. The Tire and Rim Association has established standards for rim geometry and limits on the extent to which rim flange heights may be altered. Consequently, there is a limit to the amount of load that can be redistributed from the inboard flange to the outboard flange.
The Baird et al patent also suggests increasing the section modulus of the rim by increasing the flange width and providing a wider flat contact surface. Gross changes in flange geometry, which result in an increase in the section modulus (e.g., increased flange width), have a relatively small effect on local stresses. Increased flange width also undesirably adds to wheel weight.
The present invention recognizes that the shape of the flange at the point of contact has a more significant effect on the local stresses. For a given amount of flange load, the rim flange geometry can be optimized to reduce the local stresses in the flange.
The present invention provides solutions to the above problems by providing a roll surface for contacting the ground in the absence of, or upon failure of, a tire which avoids flat surfaces and corners and includes a generous radius of curvature, or crown, at the point of contact between the rim and ground, or test dynamometer, which greatly reduces the local stresses in the area of contact. This curvature may be produced as simple radius, compound radius, elliptical shape, or other smooth curve. This local curvature can be optimized in terms of radius of curvature and location of contact to minimize the local stresses and maximize the number of cycles to failure. This feature can be applied to either inboard or outboard wheel flanges and may be used in tandem with differential flange heights.
The invention comprises, in one form thereof, a wheel having an annular rim portion of generally U-shaped cross-sectional configuration for receiving a tire and including a rim peripheral extremity functioning as a potential roll surface for contacting the ground in the event of tire failure. The extremity has a convex annular surface formed in cross-section as a smooth curve having a radius of curvature which is finite throughout. The annular surface may be formed in cross-section as a portion of an ellipse having a semi-minor axis extending generally radially along the annular rim portion. The annular surface may be formed in cross-section as a central circular segment blending smoothly into two circular segments of lesser radii at the central circular segment extremities.
An advantage of the present invention is that by reducing the intensity of the local rim flange stresses in the contact area, the test roll distance is increased and the likelihood of tire failure induced rim failure is lowered without increasing wheel weight.